1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, and, more particularly, to an exhaust gas purifying apparatus containing a NOx trap material that is disposed in an exhaust line of an internal combustion engine.
2. Description of Related Art
Lean burn engines that are advantageous to improvement of fuel efficiency are typically provided with a NOx catalyst that is disposed in an exhaust passage. The NOx catalyst contains a NOx trap material that is comprised of barium as a chief material and absorbs NOx at a lean air-fuel ratio, i.e. in an oxygen excess atmosphere and reduction-purifies and releases the absorbed NOx into an exhaust gas at a rich air-fuel ratio i.e. in an oxygen-deficient atmosphere. In a well known NOx releasing control technique such as described in, for example, Japanese Patent No. 2586739, NOx is released from the NOx trap material by making an air-fuel ratio richer when an amount of NOx absorbed by the NOx trap material that is predicted on the basis of an engine speed and/or an engine load during lean burn operation of the engine becomes as small as the NOx catalyst lowers its NOx conversion efficiency below a predetermined limit value.
However, there is the problem of sulfur-poisoning the NOx trap material. Specifically, sulfuric components such as SOx contained in fuel that the NOx trap material absorbs hinder the NOx trap material in absorbing NOx and, in consequence, lowers the NOx conversion efficiency. On account of this problem, sulfur releasing control is introduced as is well known in the art. In one of sulfur components releasing control techniques that have been well known in the art, similar to the NOx releasing control technique, sulfur is released from the NOx trap material by making an air-fuel ratio richer when an amount of sulfur components absorbed by the NOx trap material that is predicted on the basis of an amount of fuel flow and/or a temperature of the catalyst becomes lower than a predetermined amount and causing a rise in exhaust gas temperature so as to cause a rise in catalyst temperature.
There is the problem of deterioration of fuel consumption performance in making an air-fuel ratio richer aiming at releasing NOx from the NOx trap material. This problem has an adverse influence on lean burn engines that are intended to improve fuel consumption performance. It is therefore desirable to alleviate a degree of enrichment of air-fuel ratio as small as possible even though the NOx trap material is caused to release NOx.
On the other hand, the rise in exhaust gas temperature that is caused aiming at releasing sulfur components is achieved by, for example, retarding an ignition timing. However, while an engine operating condition is in a range such as, for example, a low load region and a low speed region where an exhaust gas is at a low temperature on earth and is consequently hard to rise its temperature as high as effective for the NOx trap material in releasing sulfur components (which is over 550xc2x0 C. to 600xc2x0 C.) even though how retarding an ignition timing, the NOx trap material is hard to release effectively sulfur components. There occurs rather an evil of a fall of engine torque due to retarding an ignition timing. In such an event, therefore, it is usual to wait for engine operating condition shifting to a high load region and/or a high speed region by means of postponing releasing sulfur components.
Since sulfur is contained in fuel itself as was previously described, the NOx trap material basically continues to absorb sulfur components as far as the engine is running even while the NOx trap material postpones or restricts sulfur releasing. When the NOx trap material postpones or restricts sulfur releasing for an extremely long period of time, the NOx trap material absorbs sulfur components too much to count on absorption of NOx after all. That is, the NOx trap material becomes almost impossible to absorb NOx and, in consequence, there practically occurs an event where the NOx trap material has almost no NOx therein to release. If, in such an event, it is estimated that the amount of NOx absorption has exceeded a specified amount and, as a result, an air-fuel ratio is made richer with the aim of causing the NOx trap material to release NOx, the deterioration of fuel consumption performance grows into a more serious problem.
It is an object of the present invention to provide an exhaust gas purifying apparatus that restrains an air-fuel ratio from being made rich as far as possible so as thereby to prevent a growth of deterioration of fuel consumption performance.
The aforesaid object of the present invention is accomplished by an exhaust gas purifying apparatus for purifying exhaust gases generated by an internal combustion engine that comprises a NOx trap material that is disposed in an exhaust gas passage and operative to absorb NOx in an oxygen excess exhaust gas and release the absorbed NOx when a concentration of oxygen in an exhaust gas lowers and control means for estimating an amount of NOx absorbed by the NOx trap material, for making an air-fuel ratio richer so as thereby to cause the NOx trap material to release NOx when the amount of NOx absorbed by the NOx trap material is greater than a specified amount, for estimating an amount of sulfur absorbed by the NOx trap material, and for making an air-fuel ratio richer and rising an exhaust gas temperature so as thereby to cause the NOx trap material to release sulfur when the amount of sulfur absorbed by the NOx trap material is greater than a first specified amount.
The control means conditionally restricts the sulfur releasing of the NOx trap material and restrains an air-fuel ratio from being made richer when the amount of sulfur absorbed by the NOx trap material is larger than a second specified amount that is larger than the first specified amount.
According to the exhaust gas purifying apparatus, when the NOx trap material has almost no NOx therein to release due to progress of sulfur absorption, an air-fuel ratio is restrained from being made as rich as the NOx trap material releases NOx. In consequence, even if it is estimated that the amount of NOx absorbed by the NOx trap material has exceeded the specified amount and, as a result, the air-fuel ratio is restrained from being unnecessarily made richer, so that a growth of deterioration of fuel consumption performance is prevented.
The control means may reduces a lean engine operation region when the NOx trap material continues to absorb sulfur too much to count on absorption of NOx and, however, leaves a lean engine operation region where selective reduction-purification of NOx can be counted on as its is.
When the NOx trap material becomes almost impossible to absorb NOx resulting from progress of sulfur absorption, the lean engine operation region so as thereby to restrict lean engine operation that produces NOx significantly. This restrains an emission level of NOx into the atmosphere. Furthermore, the lean engine operation region where the selective reduction-purification of NOx can be counted is not reduced so as thereby to allow lean engine operation. This prevents deterioration of fuel consumption efficiency resulting from reducing the lean engine operation region while restraining an emission level of NOx into the atmosphere.
The lean engine operation region where the selective reduction-purification of NOx can be counted is an engine operation region where a temperature of exhaust gas is comparatively low.
In this instance, there is an overlap between the engine operation region where the selective reduction-purification of NOx is enabled and an engine operation region where the sulfur releasing that is caused by retarding anignition timing. Accordingly, the NOx trap material progressively absorbs sulfur resulting from restraining sulfur releasing, so as to selectively reduce and purify NOx and discharge the purified NOx into the atmosphere while the NOx trap material reduces an amount of NOx absorption according to the sulfur absorption. This is contributory to emission control during the restraint of sulfur releasing.
The control means may retard an ignition timing so as thereby to rise a temperature of exhaust gas and restricts the NOx trap material from releasing sulfur while the engine is in an operating state where the temperature of exhaust gas does not rise to a temperature effective on sulfur releasing even when retarding the ignition timing. In addition, the control means may judge desorption properties of sulfur absorbed by the NOx trap material and restrains the sulfur releasing when an amount of non-desorbing sulfur is larger than a specified amount.
In this instance, while the engine is in an operating state where the sulfur releasing is not effectively performed, the sulfur releasing is restrained so as to alleviate a drop in engine output torque due to retardation of the ignition timing. Furthermore, a large part of sulfur absorbed in the NOx trap material is unable to desorb, the sulfur releasing is restrained so as to inhibit useless sulfur releasing, i.e. to prevent unnecessarily making an air-fuel ratio richer and rising a temperature of exhaust gas.